Field of the Invention
The present invention relates to a thermal boiler configured for obtaining energy by combusting fossil fuel or the like, and particularly relates to a lowz NOx boiler, which can reduce the discharge amount of NOx, without preparing any special or additional de-nitration apparatus.
Background Art
In general, the term “NOx” (which is discharged by the combustion of the fossil fuels or the like) means a gaseous matter containing NO and NO2. For the environmental preservation, it is necessary to reduce the discharge of such NOx gas strictly. Meanwhile, for slowing down the drying up of the oil fuel and saving the fuel cost, a low-quality fuel, containing a relatively large amount of nitrogen-containing materials, residual carbon and/or ash-forming materials, e.g., asphalt, petroleum coke, carbonized sewage sludge or the like, has been used occasionally as a boiler fuel. In the case of using such a low-quality fuel, reducing an unwanted environmental load due to exhaust combustion gas, e.g. low NOx combustion, low-soot-and-dust combustion or the like, as well as stabilized continuous operation of the boiler, e.g. positively controlling dust trouble due to combustion ash, are widely requested. In particular, in the case of using the low-quality fuel containing a relatively large amount of nitrogen and residual carbon, reducing the discharge amount of NOx, soot and dust is requested more strictly than ever before.
As one approach for reducing the NOx discharge amount of the boiler, a two-step combustion method, a combustion-gas recirculation method or the like, each utilizing the phenomenon that the NOx generation amount strongly depends on O2 partial pressure as well as on the combustion temperature, in a combustion region, has been reported. The two-step combustion method includes a first step of combusting the fuel, at a relatively high temperature, while the combustion air supplied to a burner zone is kept in a reduced condition lower than a theoretical combustion air ratio, so as to control the NOx generation amount, and a second step of further and completely combusting the fuel still remaining uncombusted, at a relatively low temperature, under an oxidative atmosphere, with excessive air separately supplied, thus substantially reducing the NOx discharge amount. In addition, the combustion-gas recirculation method includes a step of mixing or incorporating a part of the combustion gas, in a recirculation manner, into the combustion air, and then introducing high-temperature air, with lowered O2 partial pressure, into the resultant mixed gas, so as to adequately lower the flame temperature, under a slow combustion condition, thereby to effectively reduce the NOx discharge amount.
As the conventional art employing the two-step combustion technology, one structure of the low NOx boiler which was created by the inventor of this application and was described in Patent Document 1 is known. This low NOx boiler, as shown in FIG. 10, is constructed as below. A refractory material 30 is attached to an inner circumferential wall of the boiler. A high-temperature reductive combustion zone 32 is provided to a bottom portion of a combustion chamber 33, and has burners 31 mounted to a side wall thereof. The second-step combustion zone is provided above the high-temperature reductive combustion zone 32, via a narrowed portion 34, and has an inner circumferential wall formed into a water wall structure. Further, this second-step combustion zone includes second-step combustion air nozzles 35 respectively attached to a side wall thereof. Additionally, a superheater 37 is provided to an upper portion of the combustion chamber 33.
In the high-temperature reductive combustion zone 32, the fuel is combusted, at an excessive fuel concentration, with the temperature kept relatively high (e.g., approximately 1500° C. on average). Then, the combustion air is newly supplied, from the respective second-step combustion air nozzles 35, to the resultant combustion gas flowed upward into the second-step combustion zone 36 through the narrowed portion 34. In this way, the combustion of the fuel can be completed, under the oxidative atmosphere, at the relatively low temperature.
As illustrated by one experimental result shown in FIG. 3, the total amount of nitrogen-containing (or, N-containing) products, such as NO and the like, generated by the combustion, is decreased as the temperature is elevated, under a reductive atmosphere of the air ratio less than one (i.e., the air ratio<1), while being decreased as the temperature is lowered in the oxidative atmosphere of the air ratio greater than one (i.e., the air ratio>1). Namely, the two-step combustion technology described in the above Patent Document 1 utilizes this phenomenon, in order to effectively reduce the NOx generation during the combustion of the fuel containing a considerably large amount of nitrogen. More specifically, in the low NOx boiler described in the Patent Document 1, the fuel is first combusted, at the relatively high temperature, under the reductive atmosphere, in the high-temperature reductive combustion zone 32, and then the air is supplied, at the relatively low temperature, to the partly combusted gas flowed into the second-step combustion zone 36. As such, the fuel remaining uncombusted can be further combusted under the low-temperature oxidative atmosphere. Thus, the fuel combustion can be substantially completed, thereby reducing the NOx discharge amount in each zone.
In fact, such configuration of the low NOx boiler is now applied to an actual machine and serves to effectively combust or burn bunker-C, asphalt or the like. In this case, when the low-quality fuel, such as the petroleum coke, carbonized sewage sludge or the like, is used as the boiler fuel, the effect of reducing the NOx discharge amount can be achieved enough. However, since such a low-quality fuel contains a considerably large amount of ash-forming materials, produced by vanadium and the like metal, the so large amount of ash is likely to be accumulated on a furnace bottom portion, presenting a new concern.
For instance, when the low NOx boiler as disclosed in the above Patent Document 1 is operated continuously for about several to six months, a considerably large amount of ash generated by the combustion of the low-quality fuel is accumulated in the furnace. Therefore, it is necessary to periodically stop the operation of the boiler and remove such accumulated ash therefrom. However, it is quite difficult to remove, safely and securely, the ash from the furnace bottom portion, because such ash accumulated thereon is still melted at a considerably high temperature. Besides, the high-temperature reductive combustion zone surrounding the furnace bottom portion is filled with the combustion gas that is still partly combusted at the high temperature and contains poisonous gases and/or components, such as carbon monoxide, hydrogen sulfide and the like.
To effectively remove such ash, provision of a proper ash discharge port to the furnace bottom portion can be considered. However, as described above, the atmosphere in the high-temperature reductive combustion zone around the furnace bottom portion is filled with the poisonous gases heated at the high temperature. Therefore, in view of a risk of inadvertent and/or accidental gas leakage, it is rather problematic to provide the ash discharge port to such a furnace bottom portion. Further, if the ash discharge port is provided to the furnace bottom portion, the outside air may tend to enter the furnace bottom portion through the ash discharge port. This may cause negative impact on the high-temperature reductive combustion atmosphere, thus seriously deteriorating the ability for adequately performing the low NOx combustion and/or low-soot-and-dust combustion.
While the ash is melted during the fuel combustion, because of the considerably high-temperature atmosphere around the furnace bottom portion, this ash is solidified after the furnace is cooled. Therefore, for discharging such ash from the furnace bottom portion after the furnace is cooled, it is necessary to crush the ash by using a rock drill or the like. However, this operation requires frequent stop and start of the furnace as well as an unduly high cost for the maintenance.    Patent Document 1: JP2667607B